With the advent of distributed sensing in environmental monitoring, process control and in clinical applications, small, low voltage/power, low noise, and inexpensive amplifiers have begun having increasing importance. For example, certain applications require amplification of sensor signals on the order of microvolts, or even nanovolts.
Another challenge is that real world signals are analog, but current technology is becoming more and more digitally based. This raises the need of translating the analog signals to digital representations through analog to digital converters (ADCs). ADCs are expensive to build and generally do not conform with the requirements for distributed sensing of low cost, low power, and low noise.
For example, for brain-related applications such as brain machine interfaces (BMIs), the processing of small signals requires a circuit with ultra low power consumption and a small layout. Low power is needed because BMIs will generally be battery powered, while the application dictates the size of the device. With the advent of dense electrode arrays for neuron recording, interest in many such brain-related applications has greatly increased. For BMI applications, implanted electrodes in multiple cortical and sub-cortical regions are currently being used to investigate the correlation between neuron population activity and associated subject behavior in monkeys.
Ultimately, hundreds or thousands of simultaneous neural signals recorded from paraplegic humans will likely be used to control computer cursors or robot arms. Since the hardware for recording the neural signals must be located close to the implanted electrodes, the devices must be as small and lightweight as possible, particularly if many densely packed channels are to be recorded. One of the solutions to this problem is to use analog circuits to acquire and process the electrical signals transduced from implanted extracellular cortical electrodes.
Neural recordings are typically made using multiple electrodes inserted into the brain via a small craniotomy. A wire attached to a screw implanted in the skull provides a neural reference voltage. The extracellular neural signals typically have amplitudes of about 10-100 μV, and typical electrode impedances are around 100 kΩ at 1 kHz. Due to the unavoidable electrochemical effects at the electrode-tissue interface, DC offsets typically ranging from 1 to 2V generally arise across the recording electrodes. The frequencies of the brain waves of interest range from 100-400 Hz to 3 k-11 kHz, while the Local Field Potentials (LFP) extend below 1 Hz. Thus, an optimal bandpass filter for neural recording must reject the DC offset while passing the LFP signal.
The processing and transmission of the neural data poses another challenge. The most common solution currently employed is to amplify, multiplex and transmit analog intracortcical neural activity with on-chip CMOS analog circuitry, while using leads for power supply and data transfer. However, these interconnection wires can potentially cause infection in chronic implants at the points where they break the skin. On-chip analog-digital conversion is required to enhance the signal-to-noise ratio and robustness, as well as to provide a wireless transmission interface to reduce the risk of infection for chronic recording.
Conventional sigma-delta ADCs are one class of ADC. However, sigma-delta converters use oversampling techniques and complex digital circuitry at high sampling rates which consumes a large amount of power. For example, the power consumption is about 150 mW for the AD7721 from Analog Devices (Analog Devices, Norwood, Mass.) which is a CMOS 16 bit 468 kHz sigma-delta ADC. Sigma-delta ADC power consumption from other vendors are also at similar levels. Thus, lower power, lower noise, and small size integrated amplifiers which provide digital outputs are clearly needed for a variety of applications, particularly those involving low signal levels.